The invention relates to premium performance polyethylene conduits, pipe, couplings and liners. High premium performance pipe is useful under conditions which impose high stress and or high service temperatures on the pipe. The conduits may be used for example in pressure or non-pressure applications in mining, gas, waste, and potable water transportation.
The invention relates to the production of high performance conduits. The invention includes the high performance conduits, the polyethylene resin used to make them, and the process for producing the resin with properties which are required to yield the performance characteristics of the subject conduits.
The resin used in accordance with the invention exhibits a bimodal molecular weight distribution or broad molecular weight distribution and is of high molecular weight. It also has a specific distribution of comonomer within the molecular weight distribution. It is produced catalytically in a single reactor.
Increasing the molecular weight of polyethylene (and copolymers of ethylene) generally results in enhancing tensile strength, ultimate elongation, impact strength, puncture resistance, and toughness, of films and conduits produced therefrom. However, increasing the molecular weight of the polyethylene will usually decrease its processability. In addition increasing the molecular weight of the molecules that incorporate the comonomer enhances the resistance of polyethylenes to slow crack growth and rupture. By providing a mixture of a high molecular weight polyethylene with a lower molecular weight polyethylene, the desirable characteristics due to the high molecular weight component can be retained while improving processability of the mixture containing the high molecular weight component. Also providing a mixture where the comonomer incorporation is controlled further enhances the performance of these materials. To produce such mixtures, various alternatives have been considered, including post reactor or melt blending, catalysis in a single reactor with a catalyst effective to produce such a product and lastly use of multistage reactors, in which diverse average molecular weight components can be produced in each reactor.
The conduits of the invention can be fabricated to diameters in excess of 3 m.
The practical lower limit on diameters of conduits that can be manufactured with this invention is 2 to 3 millimeters.
Wall thickness can range from less than 1 millimeter to in excess of 60 millimeters. The practical range of wall thickness is dependent on the diameter of the conduit being manufactured. This is expressed as a ratio of the wall thickness to diameter, called the Standard Dimension Ratio; SDRs in the range seven to forty can be manufactured from this invention.
The long term performance of a pipe-grade resin can be evaluated by determining the time-to-failure under constant internal pressure (constant hoop stress), in a controlled environment.
The conduit of the invention sustains high stresses under conditions of elevated temperature. In accordance herewith, high stress performance is measured by hoop stress tests. Hoop stress measures the pipe burst resistance and the effect of internal pressure and internal pressure build-up on the conduit integrity. It is measured by immersing specimens of pipe in water baths at temperatures from 20 to 80xc2x0 C., typically 20, 60 and 80xc2x0 C. These specimens are then pressured to induce a hoop stress [hoop stress (MPa)=P (Dmxe2x88x92Wmin)/2 Wmin, where Wmin is the minimum pipe wall thickness in millimeters, Dm is the mean pipe outside diameter in millimeters, P is the applied pressure in megapascals in the pipe]. Typically in excess of 25 specimens are tested at each test temperature. The time to failure of each specimen is measured. The longer the life at a given hoop stress and temperature the better the performance of the material. This analysis methodology is well documented in International and National Standards, such as ISO 4437, ISO 4427, AS 4131, AS4130, ASTM D2837, and the standard method for the analysis and prediction of pipe performance as given in ISO/TR 9080 and similar publications.
Given the long lifetimes required of high performance pipe, hoop stress testing is often conducted at an elevated test temperature, such as 80xc2x0 C. The time-to-failure under constant internal pressure (constant hoop stress) at 80xc2x0 C. can be used as a measure of performance. At a test temperature of 80xc2x0 C. and a hoop stress of 5.0 MPa, the conduits of the invention exhibit a life-time in excess of 1000 hours, preferably greater than 2500 hours, most preferably greater than 4000 hours.
Resistance to impact failure is an equally important aspect of pipe performance. This was assessed by the Charpy impact test, using an instrumented impacter. Specimen dimensions were 10 mmxc3x9730 mmxc3x97160 mm, and the specimens were pre-notched with a notch to width ratio in the range of 0.2 to 0.6. Test temperatures ranged from xe2x88x9260xc2x0 C. to +60xc2x0 C. The span to width ratio of the impacter was 4. The critical strain energy release rate for failure, Gc, was calculated using the principles of linear elastic fracture mechanics. The material characteristic Gc is determined using the specimens described above. The specimen notches vary from 3 to 15 millimeters in length, generally notches of 3, 6, 9, 12, and 15 millimeters are used. At a given notch depth at least three samples are tested. The samples are held at the test temperature for two hours prior to testing. Each specimen is then placed in the instrumented impact tester and impacted. The impact tester determines the force displacement curve for the impact. These data are used to determine the energy for fracture, from the area under the force displacement curve. At a given temperature the fracture energies are plotted as a function of the notched specimen dimensions multiplied by a testing geometry factor. This plot is generally linear. The value of Gc for the material, at the temperature of interest, is the slope of the line. (Ref: J. G. Williams, Fracture Mechanics of Polymers, Ellis Horwood, Chichester, 1984.)
Conduits should be resistant to slow crack growth [Refs: ISO 4437, slow crack growth test, and D. Barry and O. Delatycki, J. Polym. Sci., B, Polym. Phys., 25, 883 (1987)] This property is measured in part by the hoop stress-life time testing cited above; however specific testing is often carried out to determine the resistance to slow crack growth. Resistance to slow crack grow was assessed using the PENT test. This test is described in ASTM 1473-94. Testing is carried out at 80xc2x0 C., and an initial tensile stress of 2.2 MPa (megapascals). The PENT test measures the time to failure of a 25xc3x9710 mm specimen which has a 3 mm sharp notch across one of its broad faces and 1 mm notches across both of its narrow faces. The specimen is tested at a constant load, and this load induces an initial tensile stress of 2.2 MPa in the specimen. Conduits of the invention exhibit PENT life times in the range of greater then 50 hours preferably greater than 100 hours, most preferably greater than 150 hours.
The conduits of the invention are produced on conventional pipe extrusion or pipe winding equipment. One of the advantages of this invention is that conduits with superior performance can be manufactured using conventional equipment. This is in part due to the ease of processing that the invention imparts due to the mixture of a low molecular weight and high molecular weight components. The processability of these materials is assessed using capillary rheometry. A capillary rheometer measures the apparent viscosity of the material at a given temperature and apparent shear rate. Conduits of the invention, exhibit in the molten state a lower apparent viscosity at a given apparent shear rate than a conventional unimodal PE pipe resin of similar melt flow index, FI at 190xc2x0 C.
The polyethylene resin of the invention is made with a bimetallic catalyst, in a single reactor, and exhibits a bimodal or broad molecular weight distribution. That is the polyethylene can be characterized as comprising at least two different polymer components which differ from each other in molecular weight. One of the polymer components has a higher molecular weight (HMW) compared to the other component of relatively lower molecular weight (LMW).
The polyethylene resin of the invention, for use in pipe production to make high performance pipe, with the impact resistance, processing advantages, resistance to slow crack growth, and hoop stress performance characteristics identified above, must satisfy four physical properties, which relate to density, FI [or I21 measured accordingly to ASTM D-1238, Condition E], the calculated weight fraction of the high molecular weight component, and calculated MI of the low molecular weight component. A brief description of the mathematical technique that is used to estimate the composition (molecular weight and weight fraction) of the bimodal molecular weight distribution appears in Computer Applications in Applied Polymer Science, ACS Symposium Series, 197, T. Provder, 45, 1982, which is expressly incorporated herein by reference. The MI of the low molecular weight component is estimated from a suitable calibration curve based on measured MIs and molecular weights of low molecular weight polyethylenes. In particular, the resins satisfy a density requirement in the range of 0.930 to 0.960 g/cc, preferably 0.935 to 0.955 g/cc, most preferably 0.940 to 0.950 g/cc. Also, the resins satisfy a FI requirement in the range of 2 to 20 dg/min., preferably 2 to 10 dg/min., most preferably 3 to 8 dg/min. Moreover, the resin must exhibit a weight fraction of the high molecular weight component, based on resin weight, of 0.20 to 0.90, preferably 0.50 to 0.80, most preferably 0.55 to 0.75. In addition, the low molecular weight component must exhibit a calculated MI of 200 to 10000 dg/min., preferably 200 to 5000 dg/min., most preferably 200 to 3000 dg/min.
The resin of this invention may be either 1) a homopolymer of ethylene; 2) at least one copolymer of a preponderance i.e., greater than 50 wt. % of ethylene with a minor amount of a 1-olefin containing 3 to about 10 carbon atoms, preferably a 1-olefin containing 4 to about 10 carbon atoms, e.g., 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-decene, and mixtures thereof; or 3) a mixture of any of the foregoing polymers. In general, the polymer product will comprise an amount of polymerized comonomer which is in the range, for example, of about 0 to 30 weight percent, based on the total weight of polymer.
The resins of the invention comprise a HMW component and a LMW component with a HMW component weight fraction which is greater than 0 (zero). The resin has a molecular weight distribution, which is characterized as MFR or Mw/Mn. The MFR of resin products of the invention can range from 20 to 300, preferably from 40 to 200, and most preferably from 50 to 150. The Mw/Mn of resin products of the invention can range from 2.5 to 60, preferably from 5 to 40, and most preferably from 10 to 30.
The compositions of the invention can be extruded into pipes and injection or blow molded into articles or extruded and blown into films. Films can be produced which are 0.2 to 10.0 mils, preferably 0.5 to 2.0 mils, thickness. Blow molded articles include bottles, containers, fuel tanks and drums.
The polyethylene resins of the invention need not be blended with other polyolefins, e.g., polyethylenes and ethylene copolymers. However, it is contemplated that the resins may be blended with other polyolefins and copolymers of e.g. ethylene such as LLPPE. Thus, the invention contemplates a composition for pipe production comprising greater than 80% by weight of the bimodal or broad molecular weight distribution polyethylene resin.
The products may contain any of various additives conventionally added to polymer compositions such as lubricants, stabilizer, antioxidants, compatibilizers, pigments, etc. These reagents can be employed to stabilize the products against degradation. For example, additive packages comprising 400-2000 ppmw hindered phenol(s); 200-2000 ppmw phosphites; 250-3000 ppmw stearates, and 0.5 to 3.0 wt. % carbon black, for addition to the resin powders, can be used for pelletization.